Corrosion Rate 2

[Joss10]

Dear mates,
Background:
A 5/6 years ago built pressure vessel (never in use), was not properly stored outdoor, some rainy water was entering inside it during long that time (5/6 years), causing localized pitting corrosion in the bottom head with the consequent metal loss.
Question:
Is it right talking about corrosion rate & remaining life in a pressure vessel never put in operation.

Any input will be appreciated.

Thanks.

 

[JohnGP]

The design of the vessel has been compromised by the corrosion, so even though it has not been in service, a fitness for service assessment needs to be performed using, for example, API 579-1/ASME FFS-1. Such assessment can be used to determine remaining life with such damage present, or indicate whether material replacement is necessary.

 

[r6155]

Joss10, see your post 22nov2016 "Remaining life.......". Please !!

Regards
r6155

 

[DSB123]

Joss10,
If corrosion has taken place then how do you know it is "fit for Service" unless you perform an assessment to API579 ?

 

[Knife44]

Why would you need to perform a FFS? If the vessel still has enough useable corrosion allowance, then operate it until you reach the minimum thickness. Fitness for service assessment is for after you have reached T-min or if you have single or isolated corrosion issues. You are literally borrowing life from the safety factor built into the vessel. Most vessels have a calculated t-min even lower than the nominal minus C.A. stamped on the data plate. I would have a full internal inspection performed on the vessel by an API 510 Pressure Vessel Inspector. They will calculate and measure the corrosion with Ultrasonic or other means and determine how much useful wall remains. I would recommend a follow up Ultrasonic survey after 1000 hours of service to calculate a short term corrosion rate.
To answer your question, a vessel is designed to hold pressure and requires a given amount of thickness. Regardless of what service it is put into and what caused the loss off thickness, it is now considered corroded. NBIC and API 510 inspection codes allow Isolated pits to be calculated differently than normal T-min.

API 510 7.4.3:
7.4.3 Evaluation of Pitting
During the current inspection, widely scattered pits may be ignored as long as all of the following are true:
a) the remaining thickness below the pit is greater than one-half the required thickness (1/2 trequired),
b) the total area of the pitting that is deeper than the corrosion allowance does not exceed 7 in.2 (45 cm2) within any
8-in. (20-cm) diameter circle,
c) the sum of the pit dimensions that is deeper than the corrosion allowance along any straight 8-in. (20-cm) line
does not exceed 2 in. (5 cm).

 

[chicopee]

A past rule of thumb which may be still in practice with newer versions of NBIC is that if the pitting is less than 50% of the wall thickness, it can be repaired by welding.

 

[Knife44]

Chicopee, I mean no disrespect by this, but I would be careful about throwing out "rules of thumb". People in the power piping and steam/boiler world have made some really bad choices by going off of what someone said instead of referencing the industry accepted standards.

Most states accept NBIC or API510. You are correct that pit dimensions are listed and determine if the pit can be accepted, ignored, and or repaired. Also, flush patches can be used to replace the entire section (Fillet weld patches are not permitted).

Also, this work shall be done by an accredited repair organization holding a valid certificate from the National Board (In some states an ASME U stamp is acceptable for repair work). A repair plate will be attached to the vessel and witnessed by the AI and a data report created and signed. Do not allow a welder, certified or not, to just jump in and start filling pits. A repair organization must be involved or the repair is considered non-code and unacceptable to most jurisdictions.

There are several methods listed here that can be used, Fitness for Service, inspection, repair etc. but applying them in the correct order is the key. You need an inspection performed before you can determine you remaining life situation and go from there.

API510,API570,API653,API577,API571,
API580, NBIC AI. A.S. Eng, BSME